// Copyright (c) .NET Foundation. All rights reserved.
// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.

using System;
using System.Globalization;
using Microsoft.AspNetCore.Http;

namespace Microsoft.AspNetCore.Routing.Constraints
{
    /// <summary>
    /// Constrains a route parameter to represent only file name values. Does not validate that
    /// the route value contains valid file system characters, or that the value represents
    /// an actual file on disk.
    /// </summary>
    /// <remarks>
    /// <para>
    /// This constraint can be used to disambiguate requests for static files versus dynamic
    /// content served from the application.
    /// </para>
    /// <para>
    /// This constraint determines whether a route value represents a file name by examining
    /// the last URL Path segment of the value (delimited by <c>/</c>). The last segment
    /// must contain the dot (<c>.</c>) character followed by one or more non-(<c>.</c>) characters.
    /// </para>
    /// <para>
    /// If the route value does not contain a <c>/</c> then the entire value will be interpreted
    /// as the last segment.
    /// </para>
    /// <para>
    /// The <see cref="FileNameRouteConstraint"/> does not attempt to validate that the value contains
    /// a legal file name for the current operating system.
    /// </para>
    /// <para>
    /// The <see cref="FileNameRouteConstraint"/> does not attempt to validate that the value represents
    /// an actual file on disk.
    /// </para>
    /// <para>
    /// <list type="bullet">  
    ///     <listheader>  
    ///         <term>Examples of route values that will be matched as file names</term>  
    ///         <description>description</description>  
    ///     </listheader>  
    ///     <item>  
    ///         <term><c>/a/b/c.txt</c></term>  
    ///         <description>Final segment contains a <c>.</c> followed by other characters.</description>  
    ///     </item>
    ///     <item>  
    ///         <term><c>/hello.world.txt</c></term>  
    ///         <description>Final segment contains a <c>.</c> followed by other characters.</description>  
    ///     </item>
    ///     <item>  
    ///         <term><c>hello.world.txt</c></term>  
    ///         <description>Final segment contains a <c>.</c> followed by other characters.</description>  
    ///     </item>
    ///     <item>  
    ///         <term><c>.gitignore</c></term>  
    ///         <description>Final segment contains a <c>.</c> followed by other characters.</description>  
    ///     </item> 
    /// </list>
    /// <list type="bullet">  
    ///     <listheader>  
    ///         <term>Examples of route values that will be rejected as non-file-names</term>  
    ///         <description>description</description>  
    ///     </listheader>  
    ///     <item>  
    ///         <term><c>/a/b/c</c></term>  
    ///         <description>Final segment does not contain a <c>.</c>.</description>  
    ///     </item>
    ///     <item>  
    ///         <term><c>/a/b.d/c</c></term>  
    ///         <description>Final segment does not contain a <c>.</c>.</description>  
    ///     </item>
    ///     <item>  
    ///         <term><c>/a/b.d/c/</c></term>  
    ///         <description>Final segment is empty.</description>  
    ///     </item>
    ///     <item>  
    ///         <term><c></c></term>  
    ///         <description>Value is empty</description>  
    ///     </item>
    /// </list>  
    /// </para>
    /// </remarks>
    public class FileNameRouteConstraint : IRouteConstraint
    {
        /// <inheritdoc />
        public bool Match(
            HttpContext? httpContext,
            IRouter? route,
            string routeKey,
            RouteValueDictionary values,
            RouteDirection routeDirection)
        {
            if (routeKey == null)
            {
                throw new ArgumentNullException(nameof(routeKey));
            }

            if (values == null)
            {
                throw new ArgumentNullException(nameof(values));
            }

            if (values.TryGetValue(routeKey, out var obj) && obj != null)
            {
                var value = Convert.ToString(obj, CultureInfo.InvariantCulture);
                return IsFileName(value);
            }

            // No value or null value.
            return false;
        }

        // This is used both here and in NonFileNameRouteConstraint
        // Any changes to this logic need to update the docs in those places.
        internal static bool IsFileName(ReadOnlySpan<char> value)
        {
            if (value.Length == 0)
            {
                // Not a file name because empty.
                return false;
            }

            var lastSlashIndex = value.LastIndexOf('/');
            if (lastSlashIndex >= 0)
            {
                value = value.Slice(lastSlashIndex + 1);
            }

            var dotIndex = value.IndexOf('.');
            if (dotIndex == -1)
            {
                // No dot.
                return false;
            }

            for (var i = dotIndex + 1; i < value.Length; i++)
            {
                if (value[i] != '.')
                {
                    return true;
                }
            }

            return false;
        }
    }
}
